This invention relates generally to seal assemblies for turbomachinery and more particularly relates to film riding seal assemblies for sealing rotor-stator gaps and the like.
Various types of turbomachinery, such as gas turbine engines, aircraft engines, and steam turbines are known and widely used for power generation, propulsion, and the like. The efficiency of the turbomachinery depends in part upon the clearances between the internal components and the leakage of primary and secondary fluids through these clearances. For example, large clearances may be intentionally allowed at certain rotor-stator interfaces to accommodate large, thermally or mechanically-induced, relative motions. Leakage of fluid through these gaps from regions of high pressure to regions of low pressure may result in poor efficiency for the turbomachinery. Such leakage may impact efficiency in that the leaked fluids fail to perform useful work.
Different types of sealing systems are used to minimize the leakage of fluid flowing through turbomachinery. The sealing systems, however, often are subject to relatively high temperatures, thermal gradients, and thermal and mechanical expansion and contraction during various operational stages that may increase or decrease the clearance therethrough. For example, traditional labyrinth seals that are assembled to run very tight clearance during a start-up transient phase might run with large clearances during steady state operations, thereby leading to poor performance at steady state operation.
One known type of seal that improves on the performance of a labyrinth seal is a “film-riding” seal in which a stator portion of the seal rides on a thin film of air, allowing it to track a rotor portion of the seal. For this type of seal to function properly it must be able to generate sufficient lift to achieve an equilibrium between the air film force and a spring force. Because of its very small thickness the air film force is very sensitive to distortion of the rotor or stator due to operating conditions (centrifugal force, uneven temperature distribution, etc.). As the relative distortion between the stator and rotor increases the load bearing capacity of the seal air film drops, and contact can take place. Prior art designs make use of rigid lifting geometry which could not conform to any distortions. This prevents the seal from generating enough force to prevent contact.
Accordingly, there remains a need for a film-riding seal that is tolerant to distortion of the rotor surface, allowing for a robust seal.